A REST API (also called a RESTful API or RESTful web API) is an application programming interface (API) that conforms to the design principles of the representational state transfer (REST) architectural style. REST APIs provide a flexible, lightweight way to integrate applications and to connect components in microservices architectures. 

At the most basic level, an API is a mechanism that enables an application or service to access a resource within another application or service. The application or service that accesses resources is the client, and the application or service that contains the resource is the server. Some APIs, such as SOAP or XML-RPC, impose a strict framework on developers.